In the field of electrophotography, and particularly in xerographic copying machines and laser printers, over recent years there have been moves towards the use of endless belt photoreceptors, rather than rigid drum photoreceptors. Endless belts can be formed by taking a long strip-like substrate, cutting it into lengths and forming each length into an endless belt by joining the two ends. Such a belt can either carry a photoconductive coating before it is seamed, or it can be coated subsequently. Alternatively, endless belts can be formed by coating a plastics material onto a cylindrical mandrel, and removing the (seamless) belt from the mandrel when the plastics material has solidified or consolidated. The plastics material, and the photoconductive layer thereon, may be formed by liquid, vapor or powder deposition techniques.
If the belt has a seam, it is not possible to form an image at the seam position and the images are arranged in fixed positions around the belt to avoid the seam. A mechanism is provided to indicate the seam position and to synchronize the images to predetermined areas or `panels` on the belt surface. In small copiers or printers such as the Xerox 5046 there may be only two such panels around the circumference of the belt but in larger machines there may be more. The Xerox 5090, for example, has seven panels.
Even if a seamless belt is used, it is often provided with synchronizing features (e.g. holes) to check the speed and position of the belt, and the images may be formed in predetermined positions in relation to these synchronizing features. Thus a copier or printer which uses a belt photoreceptor will typically have one or more synchronizing marks, such as holes, along the border of the belt, outside the imaging area, for controlling the belt during the image forming process. U.S. Pat. No. 3,912,390 discloses a copier in which a series of detectable marks on a photoreceptor belt are used to define a succession of belt panels on the belt, on each of which an image may be formed.
If one of the image forming regions is damaged or contaminated, a defective print will be produced every time this area is used and a maintenance call will be requested by the user. The number of defective prints produced may render the apparatus unusable until the photoreceptor has been repaired or replaced.
It is an object of the present invention to minimize the inconvenience caused by the need to replace a photoreceptor belt once a defect has been noticed.